1. Field of the Invention
The invention relates to speakers, and more particularly, relates to speakers constructed without permanent magnets to minimize weight.
2. Discussion of Related Art
Referring to FIG. 1, a typical speaker 10 contains a stationary support frame 12, a permanent magnet 14 mounted on the support frame 12, a movable membrane 16 covering the support frame 12, and a voice coil 18 mounted on the movable membrane 16. As is known by those skilled in the art, vibrations are induced in the voice coil 18 and membrane 16 when an alternating source signal is supplied to the voice coil 18 by an amplifier or the like and the induced magnetic field interacts with the magnetic field of the permanent magnet 14 to alternately attract and repel the voice coil 18. Such speakers work well but are relatively large and heavy since a permanent magnet may weigh as much as several pounds or more. Conventional permanent magnet speakers thus loose their attractiveness in many applications such as automobiles, airplanes, or any other application in which weight is a concern. Conventional permanent magnet speakers also begin to loose their attractiveness in applications such as high powered woofers in which the sheer size and weight of the permanent magnet required for the speaker increases the weight of the speaker beyond acceptable limits.
Magnetless speakers are known and were in fact in wide use prior to the advent of reliable permanent magnets. That is, in the early days of speaker technology when permanent magnets were not stable and had the tendency to loose their magnetism, speakers were manufactured with a continuously powered stationary electromagnet. Referring to FIG. 2, such speakers 20 include a frame 22, an electromagnet 24 mounted on the frame 22, a movable membrane 26 mounted on the frame 22, and a voice coil 27 mounted on the membrane 26 and movable therewith. The coil 28 of electromagnet 24 or field coil is continuously powered by an internal DC power source 29 such as a battery to produce a constant-polarity magnetic field for interaction with the alternating field in voice coil 27. Speakers employing permanent electromagnets were not only heavy and cumbersome because they required their own internal power source, but were also very inefficient and were replaced by permanent magnet speakers as soon as permanent magnet technology was suitably developed. The electromagnets of such speakers, being powered by an independent power sources, also were necessarily not excited in proportion to the source signals exciting the voice coils.